1. Field of the Invention
This invention concerns a process for converting carboxylic acid amides to the corresponding carboxylic acid esters by reacting a vaporized mixture of the amides and lower aliphatic alcohols in the presence of a suitable catalyst. More particularly, the process is directed to the conversion of methacrylamide to methyl methacrylate.
2. Relation to the Prior Art
1. U.S. Pat. No. 2,913,486 claims a one-step process of reacting an unsaturated aliphatic nitrile, specifically acrylonitrile or methacrylonitrile, with a lower aliphatic alcohol and a molar amount of water in the vapor phase over a solid esterification catalyst at 400.degree.-800.degree. F. (204.degree.-427.degree. C.), preferably at 600.degree.-650.degree. F. (315.degree.-344.degree. C.) to produce methyl acrylate or methyl methacrylate. The solid esterification catalysts disclosed are silica, silica-alumina, titania, thoria, and zirconia. The catalyst activity may be enhanced by small amounts of phosphoric or molybdic acid, and the catalyst may include promoters such as platinum, nickel, iron, copper and silver. The preferred catalyst is silica.
2. U.S. Pat. No. 3,466,320 claims a one-step process of preparing a lower alkyl ester of an .alpha.,.beta.-unsaturated acid from the corresponding .alpha.,.beta.-unsaturated nitrile, water, and a lower alkanol in the vapor phase in the presence of a solid esterification catalyst containing 1% of niobium pentoxide at temperatures of 150.degree.-500.degree. C. Preferred catalysts are silica, alumina, titanium dioxide, zirconia, and mixtures thereof, and preferred reactants are acrylonitrile, methacrylonitrile, methanol and ethanol.
3. Japanese Patent Publication (Kokai) 72-25, 120 discloses a vapor-phase process for converting acrylonitrile, methanol and water to methyl acrylate in the presence of solid catalysts at 250.degree. C. Mixtures of boric oxide with alumina, titania, silica, zinc oxide and zirconia are disclosed as suitable catalysts.
4. The references listed herein below relate to the conversion of carboxylic acid amides and nitriles to esters by liquid phase processes in mineral acid media such as sulfuric and phosphoric acids. Unsaturated nitriles such as methacrylonitrile may be generated in situ from ketone cyanohydrins such as acetone cyanohydrin. In the presence of stoichiometric amounts of added water, these nitriles are converted to the corresponding amides which may be isolated before they are reacted with alcohols to form ester products. During such reactions, the mineral acid is consumed stoichiometrically and converted to the ammonium salt of the acid by the ammonia liberated.
Such processes are disclosed in:
(a) A. Bonz. Zeitschr. physik. Chem., (A), 2, 865-900 (1888);
(b) L. Meyer, Ber., 22, 24 (1889);
(c) U.S. Pat. Nos. 3,332,984 and 3,406,120.
5. Other processes in solution which have been used to convert amides to esters are disclosed in the following references:
(a) Belgian Pat. No. 843,436 (West German equivalent, Auslegeschrift No. 2528524) claims a process of preparation of carboxylic esters from the corresponding amides using a metal carboxylate catalyst;
(b) French Pat. No. 1,161,140 (West German equivalent, Auslegescrift, No. 1,071,689) claims a process of making aromatic carboxylic esters from the corresponding amides using an alkaline catalyst;
(c) E. H. White, J. Amer. Chem. Soc., 76, 4497 (1954) and 77, 6011 (1955) describes the conversion of a secondary amide to an ester by N-nitrosation and thermal decomposition according to the reaction: ##STR1##
(d) J. S. Matthews and J. P. Cookson, J. Org. Chem., 34, 3204 (1969) describe the reaction of amides with alkyl halides in aqueous media to produce esters according to the reaction: EQU RX+R'CONR.sub.2 "+H.sub.2 O.fwdarw.R'CO.sub.2 R+NR.sub.2 "H.sub.2 X
(e) D. H. R. Barton, A. L. J. Beckwith and A. Goosen, J. Chem. Soc., 181 (1965) describe the photolysis of N-iodoamides to lactones (internal esters): ##STR2##
None of the above-listed references discloses a process for converting carboxylic acid amides to carboxylic acid esters by reacting the amides and lower aliphatic alcohols in the vapor phase in the presence of the catalysts disclosed hereinbelow at 100.degree. C. to 400.degree. C.